The Polikarpov Po-2 (also U-2) served as a general-purpose Sovietbiplane, nicknamed Kukuruznik (Russian: Кукурузник, from Russian "kukuruza" (кукуруза) for maize; thus, "maize duster" or "crop duster"), NATO reporting name "Mule".[N 1] The reliable, uncomplicated concept of the Po-2's design made it an ideal training aircraft, as well as doubling as a low-cost ground attack, aerial reconnaissance, psychological warfare and liaison aircraft during war, proved to be one of the most versatile light combat types to be built in the Soviet Union.[3] As of 1978 it remained in production for a longer period of time than any other Soviet-era aircraft.[3]

It is one of the most produced aircraft, and may be the most produced biplane, in the history of aviation. More than 40,000 Po-2s may have been built between 1928 and 1953.[citation needed] However, production figures for Polikarpov U-2 and Po-2 bombers and trainers combined are as low as 20,000, up to 30,000.[2] with production ending as soon as 1952.[2] Correct figures are hard to come by since low-rate production by small repair shops and air clubs likely continued until 1959.[4]

The aircraft was designed by Nikolai Polikarpov to replace the U-1 trainer (Avro 504), itself known as Avrushka to the Soviets. Its name was changed to Po-2 in 1944, after Polikarpov's death, according to the new Soviet naming system, usually using the first two letters of the designer's family name, or the design bureau that created it.

The prototype of the U-2, powered by a 74 kW (99 hp) Shvetsov M-11 air-cooled five cylinder radial engine, first flew on 7 January 1928 piloted by M.M. Gromov. Aircraft from the pre-production series were tested at the end of 1928 and serial production started in 1929 in Factory Nr 23 in Leningrad. Production in the Soviet Union ended in 1953, but license-built CSS-13 were still produced in Poland until 1959.

A damaged and abandoned Po-2 forced to land in Ukraine, and subsequently captured by German troops, 1941.

From the beginning, the U-2 became the basic Soviet civil and military trainer aircraft, mass-produced in a "Red Flyer" factory near Moscow. It was also used for transport, and as a military liaison aircraft, due to its STOL capabilities. Also from the beginning it was produced in an agricultural aircraft variant, what earned it its nickname Kukuruznik. Although entirely outclassed by contemporary aircraft, the Kukuruznik served extensively on the Eastern Front in World War II, primarily as a liaison, medevac and general supply aircraft. It was especially useful for supplying Soviet partisans behind the front line. Its low cost and easy maintenance led to a production run of over 40,000. Manufacturing of the Po-2 in the USSR ceased in 1949, but until 1959 a number were assembled in Aeroflot repair workshops.

First trials of arming the aircraft with bombs took place in 1941.

During the defence of Odessa, in September 1941, the U-2 was used as a reconnaissance aircraft and as a light, short-range, bomber. The bombs, dropped from a civil aircraft piloted by Pyotr Bevz, were the first to fall on enemy artillery positions.[5] From 1942 it was adapted as a light night ground attack aircraft.

Nikolay Polikarpov supported the project, and under his leadership, the U-2VS (voyskovaya seriya - Military series) was created. This was a light night bomber, fitted with bomb carriers beneath the lower wing, to carry 50 or 100 kg (110 or 220 lbs) bombs up to a total weight of 350 kg (771 lb) and armed with ShKAS or DA machine guns in the observer's cockpit.[5]

Wehrmacht troops nicknamed it Nähmaschine (sewing machine) for its rattling sound and Finnish troops called it Hermosaha (Nerve saw) as the Soviets flew nocturnal missions at low altitudes: the engine had a very peculiar sound, which was described as nerve-wracking, therefore the name. The enemy soon became aware of the threat posed by the U-2, and Luftwaffe pilots were given special instructions for engaging these aircraft. The material effects of these missions may be regarded as insignificant, but the psychological effect on German troops was much more noticeable. They typically attacked by complete surprise in the dead of night, denying German troops sleep and keeping them constantly on their guard, contributing yet further to the already exceptionally high stress of combat on the Eastern front. Their usual tactics involved flying only a few meters above the ground, rising for the final approach, cutting off the engine and making a glidingbombing run, leaving the targeted troops with only the eerie whistling of the wind in the wings' bracing-wires as an indication of the impending attack. Luftwaffe fighters found it extremely hard to shoot down the Kukuruznik because of three main factors: the rudimentary aircraft could take an enormous amount of damage and stay in the air, the pilots used the defensive tactic of flying at treetop level, and the stall speed of both the Messerschmitt Bf 109 and the Focke-Wulf Fw 190 was similar to the Soviet aircraft's maximum cruise speed, making it difficult for the newer aircraft to keep a Po-2 in weapons range for an adequate period of time.[6] The success of the Soviet night harassment units using the Po-2 inspired the Luftwaffe to set up similar Störkampfstaffel "harassment combat squadrons" on the Eastern Front using their own obsolete 1930s-era, open cockpit biplane and parasol monoplane aircraft, eventually building up to larger Nachtschlachtgruppe (night attack group) units of a few squadrons each.

The U-2 was known as the aircraft used by the 588th Night Bomber Regiment, composed of an all-women pilot and ground crew complement. The unit became notorious for daring low-altitude night raids on German rear-area positions. Veteran pilots, Yekaterina Ryabova and Nadezhda Popova on one occasion flew eighteen such missions in a single night. The women pilots observed that the enemy suffered a further degree of demoralization simply due to their antagonists being female. As such, the pilots earned the nickname "Night Witches" (German Nachthexen, Russian Ночные Ведьмы/Nočnye Ved’my). The unit earned numerous Hero of the Soviet Union citations and dozens of Order of the Red Banner medals; most surviving pilots had flown nearly 1,000 combat missions by the end of the war and had taken part in the Battle of Berlin.

The Polish Air Force used these slow and manoeuvrable aircraft for air reconnaissance and COIN operations against UPA bands in mountainous area of Bieszczady. Pilots and navigators were dispatched to look for concentrations of UPA forces and if needed, engage them with machine guns and grenades. On several occasions, the UPA managed to bring down some of the Po-2s, but never captured or used one, as pilots were ordered to burn their aircraft in case of a crash.[7]

North Korean forces used the Po-2 in a similar role in the Korean War. A significant number of Po-2s were fielded by the Korean People's Air Force, inflicting serious damage during night raids on Allied bases.[8] On 28 November, at 0300 hours, a lone Po-2 attacked Pyongyang airfield in northwestern Korea. Concentrating on the 8th Fighter-Bomber Group's parking ramp, the Po-2 dropped a string of fragmentation bombs squarely across the Group's lineup of F-51 Mustangs. Eleven Mustangs were damaged, three so badly that they were destroyed when Pyongyang was abandoned several days later.

On 17 June 1951, at 01:30 hours, Suwon Air Base was bombed by two Po-2s. Each biplane dropped a pair of fragmentation bombs. One scored a hit on the 802nd Engineer Aviation Battalion's motor pool, damaging some equipment. Two bombs burst on the flight line of the 335th Fighter Interceptor Squadron. One F-86A Sabre (FU-334 / 49-1334) was struck on the wing and began burning. The fire took hold, gutting the aircraft. Prompt action by personnel who moved aircraft away from the burning Sabre preventing further loss. Yet eight other Sabres had been damaged in the brief attack, four seriously. One F-86 pilot was among the wounded. The North Koreans subsequently credited Lt. La Woon Yung with this damaging attack.[9]

UN forces named the Po-2's nighttime appearance Bedcheck Charlie and had great difficulty in shooting it down — even though night fighters had radar as standard equipment in the 1950s, the wood-and-fabric-construction of the Po-2 gave only a minimal radar echo, making it hard for an opposing fighter pilot to acquire his target. As Korean war U.S. veteran Leo Fournier remarks about "Bedcheck Charlie" in his memoirs later on: "... no one could get at him. He just flew too low and too slow." On 16 June 1953, a USMCAD-4 from VMC-1 piloted by Major George H. Linnemeier and CWO Vernon S. Kramer shot down a Soviet-built Polikarpov Po-2 biplane, the only documented Skyraider air victory of the war. One Lockheed F-94 Starfire was lost while slowing to 110 mph during an intercept of a Po-2 biplane.[10]

U-2: Basic model, built in large numbers as a two-seat primary trainer. It was also built in many different versions, both as civil and military aircraft. The U-2 variants also included a light transport, utility, reconnaissance and training aircraft. Power plant was the M-11 radial piston engine of 75 kW (100 hp). Later models were also equipped with uprated M-11 engines of 111 kW (150 hp). Some aircraft were fitted with a rear closed cabin, other were fitted with sledges or floats.

U-2AP: Agricultural aircraft, with a rear cab replaced with a container for 200–250 kg (441-551 lb) of chemicals. 1,235 were built in 1930–1940.

U-2G: This experimental aircraft had all the controls linked to the control column. One aircraft only.

U-2KL: Two aircraft fitted with a bulged canopy over the rear cabin.

U-2LSh: Two-seat ground-attack, close-support aircraft. The aircraft were armed with one 7.62 mm (0.30 in) ShKAS machine-gun in the rear cockpit. It could also carry up to 120 kg (265 lb) of bombs and four RS-82 rockets. Also known as the U-2VOM-1.

U-2LPL: Experimental prone-pilot research aircraft.

U-2M: This floatplane version was fitted with a large central float and two small stabilizing floats. Not built in large numbers. Also known as the MU-2.

U-2P: Floatplane version, built only in limited numbers, in several variants with different designations.

U-2S: Air ambulance version, built from 1934. It could take a physician and an injured on a stretcher on a rear fuselage, under a cover. Variant U-2S-1 from 1939 had a raised fuselage top upon the stretcher. From 1941 there were also used two containers for stretchers, that could be fitted over lower wings or two containers for two seating injured each, fitted under lower wings.

U-2SS: Air ambulance aircraft.

U-2ShS: Staff liaison version, built from 1943. It had a wider fuselage and a closed 4-place rear cab.

U-2SP: Civil transport version, could carry two passengers in open individual cabs, built from 1933. Other roles included aerial survey, and aerial photography. A total of 861 were built between 1934 and 1939.

U-2SPL: This limousine version was fitted with rear cabin for two passengers.

U-2LNB: Soviet Air Force night attack version, built from 1942. Armed with one 7.62 mm (0.30 in) ShKAS, plus up to 250 kg of bombs under the wings for land support. Earlier aircraft were converted to improvised bombers from 1941.

Po-2S-3: Post-war ambulance version, which had two underwing containers, each one was designed to transport one stretcher patient. Also known as the Po-2SKF.

Po-2ShS: Staff communications aircraft, fitted with an enclosed cabin for the pilot and two or three passengers.

Po-2SP: Post-war aerial photography, geographic survey aircraft.

RV-23: This floatplane version of the U-2 was built in 1937. It was used in a number of seaplane altitude record attempts. The RV-23 was powered by a 529 kW (710 hp) Wright R-1820-F3 Cyclone radial piston engine.

CSS-13: Polish licence version, built in Poland in WSK-Okęcie and WSK-Mielec after World War II (about 500 built in 1948–1956).

CSS S-13: Polish ambulance version with a closed rear cab and cockpit and Townend ring (53 built in WSK-Okęcie in 1954-1955, 38 converted to S-13).

E-23: Research version, built in the Soviet Union in 1934, for research into inverted flight.

Piloted by the fictional Ludmila Gorbunova, the Po-2 is featured (as the U-2) in the Harry Turtledove alternate history series Worldwar as one of the few examples of human machinery that has managed to evade destruction from a technologically superior invading alien force. Because of the Po-2's wooden construction, low altitude, and slow speed, the aliens have an extremely hard time detecting it or shooting it down.